THE MINERALOGRAPHY OF THE FELDSPARS 209 
If the figures are reliable there should be some basis for the 
opinion that the two similar minerals may be isomeric forms of 
the same substance. 
TABLE I 
Mineral Melting-Point Latent Heat of Fusion Specific Heat 
Orthoclase.......... 1200° roo cals. 0.1877 
Microcline........ ay | I17o° 83 cals. 0.107* 
*See W. P. White, Amer. Jour. Sci. (4), Vol. XLVII (January, 1919), p. 17, for more modern values 
of the specific heat of microcline. 
Harker says: 
If orthoclase and microcline are dimorphous, the latter must clearly be 
the lower [temperature] form. Where it occurs with the apparent characters 
of a primary mineral it is the latest product of crystallization and is charac- 
teristic of the most acid of granites and especially of pegmatites..... The 
conversion of orthoclase to microcline, or the setting up of microcline structure 
in orthoclase, has been attributed to dynamic causes." 
Another explanation of the microclinic texture in potash feld- 
spar is that it is solely due to stresses set up by dynamic forces and 
is not explained by the theory of dimorphism. Thus Rosenbusch’ 
remarks: 
The fact that microcline is almost wholly confined to the older eruptive 
rocks which have been subjected to processes of faulting and pressure, together 
with the observation that normal orthoclase assumes the microstructure of 
microcline when it has experienced strong pressure, leads to the supposition 
that microcline-structure is a pressure phenomenon. ‘The correctness of this 
assumption can be verified in many cases. The occurrence of crystals of 
microcline in cavities, however, proves that it has not been produced in this 
way in all cases. Still the absence of microcline from unaltered extrusives is 
notable. 
The writer is fully aware that microclinic texture is frequently 
the result of dynamic stresses, but is inclined to the view that the 
stresses of dynamic metamorphism permit orthoclase, which is 
metastable at temperatures below its inversion point, to change to 
microcline. His conclusion is that pressure does not produce micro- 
cline from orthoclase; it only initiates and accelerates the change. 
t Alfred Harker, Natural History of Igneous Rocks (1909), p. 259- 
2 Rosenbusch-Iddings, Microscopical Physiography of Rock-Forming Minerals, 
P- 320. : 
